Fusee
Encyclopedia
Used in antique spring
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

-powered mechanical watch
Mechanical watch
A mechanical watch is a watch that uses a mechanical mechanism to measure the passage of time, as opposed to modern quartz watches which function electronically. It is driven by a spring which must be wound periodically...

es and clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

s, a fusee is a cone
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...

-shaped pulley
Pulley
A pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...

 with a spiral groove around it, wound with a cord or chain which is attached to the mainspring
Mainspring
A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

 barrel
Barrel (horology)
Used in mechanical watches and clocks, a barrel is a cylindrical metal box closed by a cover, with a ring of gear teeth around it, containing a spiral spring called the mainspring, which provides power to run the timepiece. The barrel turns on an arbor . The spring is hooked to the barrel at its...

. Fusees were used from the 15th century to the early 20th century to improve timekeeping by equalizing the uneven pull of the mainspring as it ran down.




Origin

The origin of the fusee is not known. Many sources erroneously credit Jacob Zech of Prague with inventing it around 1525, but it actually appeared with the first spring driven clocks in the 15th century. The idea probably did not originate with clockmakers, since the earliest known example is in a crossbow
Crossbow
A crossbow is a weapon consisting of a bow mounted on a stock that shoots projectiles, often called bolts or quarrels. The medieval crossbow was called by many names, most of which derived from the word ballista, a torsion engine resembling a crossbow in appearance.Historically, crossbows played a...

 windlass
Windlass
The windlass is an apparatus for moving heavy weights. Typically, a windlass consists of a horizontal cylinder , which is rotated by the turn of a crank or belt...

 in a 1405 military manuscript. Drawings from the 15th century by Filippo Brunelleschi
Filippo Brunelleschi
Filippo Brunelleschi was one of the foremost architects and engineers of the Italian Renaissance. He is perhaps most famous for inventing linear perspective and designing the dome of the Florence Cathedral, but his accomplishments also included bronze artwork, architecture , mathematics,...

  and Leonardo da Vinci
Leonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...

 (left) show fusees. The earliest existing clock with a fusee, also the earliest spring-powered clock, is the Burgunderuhr (Burgundy clock), a chamber clock whose iconography suggests that it was made for Phillipe the Good, Duke of Burgundy about 1430, now in the Germanisches Nationalmuseum
Germanisches Nationalmuseum
The Germanisches Nationalmuseum is a museum in Nuremberg, Germany. Founded in 1852, houses a large collection of items relating to German culture and art extending from prehistoric times through to the present day...

  The earliest definitely dated fusee clock was made by Zech in 1525. The word fusee comes from the French fusée and late Latin fusata, 'spindle full of thread'.

How it works

The mainspring
Mainspring
A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

 is coiled around a stationary axle (arbor
Arbor
Arbor or arbour may refer to:*Arbor , a landscape structure*Arbor or mandrel*Arbor, California*Arbor, a counterweight-carrying device found in theater fly systems...

), inside a cylindrical box, the barrel
Barrel (horology)
Used in mechanical watches and clocks, a barrel is a cylindrical metal box closed by a cover, with a ring of gear teeth around it, containing a spiral spring called the mainspring, which provides power to run the timepiece. The barrel turns on an arbor . The spring is hooked to the barrel at its...

. The force of the spring turns the barrel. In a fusee clock, the barrel turns the fusee by pulling on the chain, and the fusee turns the clock's gears.
  1. When the mainspring is wound up (Fig. 1), all the chain is wrapped around the fusee from bottom to top, and the end going to the barrel comes off the narrow top end of the fusee. So the strong pull of the wound up mainspring is applied to the small end of the fusee, and the torque on the fusee is reduced by the small lever arm of the fusee radius.
  2. As the clock runs, the chain is unwound from the fusee from top to bottom and wound on the barrel.
  3. As the mainspring runs down (Fig. 2), more of the chain is wrapped on the barrel, and the chain going to the barrel comes off the wide bottom grooves of the fusee. Now the weaker pull of the mainspring is applied to the larger radius of the bottom of the fusee. The larger lever arm compensates for the weaker force of the spring, keeping the drive force constant.
  4. To wind the clock up again, a key is fitted to the protruding squared off axle (winding arbor) of the fusee and the fusee is turned. The pull of the fusee unwinds the chain off the barrel and back onto the fusee, winding the mainspring.

The gear on the fusee drives the movement's wheel train
Wheel train (horology)
In horology, a wheel train is the gear train of a mechanical watch or clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology for their gear trains which is not used in other applications of gears.Watch...

, usually the center wheel. There is a ratchet
Ratchet (device)
A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...

 between the fusee and its gear (not visible, inside the fusee) which prevents the fusee from turning the clock's wheel train backwards while it is being wound up. In quality watches and many later fusee movements there is also a maintaining power
Maintaining power
In horology, a maintaining power is a mechanism for keeping a clock or watch going while it is being wound.-Huygens:The weight drive used by Christiaan Huygens in his early clocks acts as a maintaining power...

spring, to provide temporary force to keep the movement going while it is being wound. This type is called a going fusee. It is usually a planetary gear mechanism (epicyclic gearing
Epicyclic gearing
Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear...

) in the base of the fusee "cone") which then provides turning power in the opposite direction to the 'winding up' direction therefore keeping the watch or clock running during winding.

Most fusee clocks include a 'winding stop' mechanism to prevent the mainspring and fusee from being wound up too far, possibly breaking the chain. As it is wound, the fusee chain rises toward the top of the fusee. When it reaches the top, it presses against a lever, which moves a metal blade into the path of a projection sticking out from the edge of the fusee. As the fusee turns, the projection catches on the blade, preventing further winding.

History

Springs
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

 were first employed to power clocks in the 15th century, to make them smaller and portable. These early spring-driven clocks were much less accurate than weight-driven clocks. Unlike a weight on a cord, which exerts a constant force to turn the clock's wheels, the force a spring exerts diminishes as the spring unwinds. These clocks' primitive verge and foliot timekeeping mechanism was sensitive to changes in drive force. So spring-driven clocks slowed down over time as the mainspring
Mainspring
A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

 unwound. This problem is called lack of isochronism
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

.

Two solutions to this problem appeared with the first spring driven clocks; the stackfreed and the fusee. The stackfreed, a crude cam
Cam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...

 compensator, added a lot of friction and was abandoned after less than a century. The fusee was a much more lasting idea. As the movement ran, the tapering shape of the fusee pulley continuously changed the mechanical advantage
Mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...

 of the pull from the mainspring, compensating for the diminishing spring force. Clockmakers apparently empirically discovered the correct shape for the fusee, which is not a simple cone but a hyperboloid. The first fusees were long and slender, but later ones have a more squat compact shape. Fusees became the standard method of getting constant force from a mainspring, used in most spring-wound clocks, and watch
Watch
A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...

es when they appeared in the 17th century.

At first the fusee cord was made of gut
Catgut
Catgut is a type of cord that is prepared from the natural fibre found in the walls of animal intestines. Usually sheep or goat intestines are used, but it is occasionally made from the intestines of cattle, hogs, horses, mules, or donkeys.-Etymology:...

, or sometimes wire. Around 1650 chains began to be used, which lasted longer. Gruet of Geneva is widely credited with introducing them in 1664, although the first reference to a fusee chain is around 1540. Fusees designed for use with cords can be distinguished by their grooves, which have a circular cross section, where ones designed for chains have rectangular-shaped grooves.

Around 1726 John Harrison
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...

 added the maintaining power
Maintaining power
In horology, a maintaining power is a mechanism for keeping a clock or watch going while it is being wound.-Huygens:The weight drive used by Christiaan Huygens in his early clocks acts as a maintaining power...

 spring to the fusee to keep marine chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

s running during winding, and this was generally adopted.

Obsolescence

The fusee was a good mainspring compensator, but it was also expensive, difficult to adjust, and had other disadvantages:
  • It was bulky and tall, and made pocket watches unfashionably thick.
  • If the mainspring broke and had to be replaced, a frequent occurrence with early mainsprings, the fusee had to be readjusted to the new spring.
  • If the fusee chain broke, the force of the mainspring sent the end whipping about the inside of the clock, causing damage.

Isochronism
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

 was recognised as a serious problem throughout the 500 year history of spring-driven clocks. Many parts were gradually improved to mitigate this problem, until finally the fusee was made unnecessary in most timepieces.

The invention of the pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

 and the balance spring
Balance spring
A balance spring, or hairspring, is a part used in mechanical timepieces. The balance spring, attached to the balance wheel, controls the speed at which the wheels of the timepiece turn, and thus the rate of movement of the hands...

 in the mid-17th century made clocks and watches much more isochronous, by making the timekeeping element a harmonic oscillator
Harmonic oscillator
In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x: \vec F = -k \vec x \, where k is a positive constant....

, with a natural 'beat' resistant to change. The pendulum clock
Pendulum clock
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...

 with an anchor escapement
Anchor escapement
In horology, the recoil or anchor escapement is a type of escapement used in pendulum clocks. An escapement is the mechanism in a mechanical clock that maintains the swing of the pendulum and allows the clock's wheels to advance a fixed amount with each swing, moving the hands forward...

, invented in 1670, was sufficiently independent of drive force that only a few had fusees.

In pocketwatches, the verge escapement
Verge escapement
The verge escapement is the earliest known type of mechanical escapement, the mechanism in a mechanical clock that controls its rate by advancing the gear train at regular intervals or 'ticks'. Its origin is unknown. Verge escapements were used from the 14th century until about 1800 in clocks...

, which required a fusee, was gradually replaced by escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

s that were less sensitive to changes in mainspring force. In 1760, Jean-Antoine Lépine
Jean-Antoine Lépine
Jean-Antoine Lépine , was a revolutionary and inventive French watch and clockmaker, who contributed with crucial inventions for watchmaking still used nowadays...

 dispensed with the fusee, inventing a going barrel to power the watch gear train directly. This contained a very long mainspring, of which only a few turns were used to power the watch. Thus, only a part of the mainspring's 'torque curve' was used, where the torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

 was approximately constant. In the 1780s, pursuing thinner watches, French watchmakers adopted the going barrel with the cylinder escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

. By 1850, the Swiss and American watchmaking industries employed the going barrel exclusively, aided by new methods of adjusting the balance spring
Balance spring
A balance spring, or hairspring, is a part used in mechanical timepieces. The balance spring, attached to the balance wheel, controls the speed at which the wheels of the timepiece turn, and thus the rate of movement of the hands...

 so that it was isochronous. England alone continued to make the bulkier full plate fusee watches until about 1900. After this, the only remaining use for the fusee was in marine chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

s, where the highest precision was needed, until they became obsolete in the 1970s.

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